Most modern common rail fuel systems utilize an electronically controlled high pressure pump to control fuel pressure in a common rail that is fluidly connected to a plurality of fuel injectors. Effective control of an engine utilizing a common rail fuel system centers on precise control over rail pressure, fuel injection timings and fuel injection quantities. Most manufacturers utilize some form of a feedback control strategy, such as a proportional integrator derivative (PID) controller to control output of a high pressure pump to in turn control fuel pressure in the common rail. For instance, U.S. Pat. No. 5,507,266 teaches a PID controller to control pressure in a common rail fuel system for a spark ignited engine. In almost all common rail fuel systems, the high pressure pump is driven directly by the engine, but output of the pump may be controlled either by some variable displacement strategy, spill control valves and even an inlet metered strategy. Depending upon the particular high pressure pump strategy chosen, different control issues can arise as different pump hardware behave differently in real engine applications. For instance, some pumps may be less expensive, but may exhibit more hysteresis than other pump hardware.
Finding a good balance between cost and effective rail pressure control can further be complicated by recent improvements to fuel injectors to improve efficiency. In many cases, electronically controlled fuel injectors utilize liquid fuel not only as an injection medium, but also as a control fluid. The fuel utilized to control operation of the fuel injector is typically returned to tank for recirculation during injection events. Between injection events, manufacturers have sought to improve efficiency by reducing leakage in fuel injectors through a variety of strategies, including better control valve seating and tighter tolerances in guide clearances separating high pressure areas from low pressure areas within the fuel injector. As fuel injectors become better at avoiding leakage, rail pressure control problems can become acute because common rail supply pumps can only control pressure by changing the pump output to be greater or less, but cannot remove fuel from the common rail in order to lower pressure in the same. Thus, depending upon the specific hardware in a common rail fuel system, providing a cost effective strategy for precisely controlling rail pressure can be problematic.
The present disclosure is directed toward one or more of the problems set forth above.